If you go for the "battles" approach that Gulla says, you can easily adapt the viking raid in TOME to fit your story arc in a more divine-infernal approach. THey can be bandits. No need for a river, even if you can have plenty of them around as well. One option would be to adapt the monastery ruins to form the basis for a village that has developed since it was abandoned by the monks.
I like the "reconnecting the groves" idea 
Nice one.
TO make stuff more interesting I would NOT tell the player that the second aura to emerge is infernal (hermetics cannot detect it as such after all. Demons can masquerade as other things, so they might put forward an appearance of messed up angels/saints/monks. Maybe divine, maybe faerie. Or maybe he tries to convince your character that he is the actual forest spirit and the other one is just a faerie trying to take over his rightful domain. Now, if you could make some sacrifices to empower him...
Cheers,
Xavi